The Universe is infinitely big. We are infinitely small. As such, there is no reason to believe that we have any Grand Purpose, because we cannot possibly affect anything on the grand scale of the Universe.
We have all of the basic functions and needs of other species of animals. (ie. breathing, eating, homeostasis, sex etc.) We differ from other animals in that we are more intelligent and we are capable of reason. We were not always so - we just evolved and became gradually smarter.
This is why I believe we would have to be arrogant to assume that us humans have some Grand Purpose to fulfill. However, do not confuse the idea of a Grand Purpose (such as being an instrument in some divine plan or some other nonsense) with that of a purpose that we set for ourselves.
Neither I nor Stephen (if I understand him correctly) are denying the latter.
For myself, I'm just saying that we're not meant to do anything. That there is no grand scheme of things in which we are to play a certain part. That no god created us for any specific reason.
Rather, by our actions, we ourselves create a reason, a purpose for our existence.
Of course, I have nothing against anyone believing that they're a part of some divine plan. It may even be true -- we may all be a part of some divine plan. But that this should actually be true - is extremely unlikely.